This invention relates to a method for recovering energy raw materials from a subterranean formation by the introduction of water/oxygen slurries into the formation.
The techniques used in recovering raw energy materials from subterranean formations varies depending on such factors as the form of energy raw material, geology, financial resources, etc. In oil production, the most common approach uses a "primary recovery" phase of 3 to 5 years after drilling a well. In primary recovery no effort is made to increase production beyond the energy raw material that is readily extracted due to pumping or pressure within the formation. Secondary recovery generally involves mobilizing additional oil by pumping water through the formation. Primary and secondary recovery leave large amounts of oil in the ground (approximately 65% to 80%).
Tertiary recovery is done by several methods, such as in-situ combustion and thermal displacement. The invention of the in-situ combustion method for petroleum recovery by F.A. Howard in 1923, did not yield substantial recoveries until recently due to control problems and the unpredictability of the method. This in-situ combustion method produces sufficient heat within a petroleum reservoir which, by means of partial combustion of the oil residues in the petroleum reservoir, enable the recovery of the remaining oil. The amount of combustion heat released in a reaction between oxygen and organic fuels is on average 3,000 kcal. per Kg oxygen. The important processes contributing to petroleum displacement are viscosity reduction by means of heat, distillation and cracking (i.e., "thinning") and extraction of the oil by means of miscible products. This is similar to the method specified in U.S. Pat. No. 3,026,935.
The use of oxygen gas to create an in situ burn has drawbacks. Its reactivity in higher purities can cause fires and explosions. The handling of compressed oxygen flowing through piping systems requires special precautions which have been developed. Such precautions include the use of large inner surfaces in relation to volume, appropriate geometry to prevent local temperature peaks, and lower purity oxygen content (because oxygen at 95% purity can ignite steel, though the burn is not self-sustaining). High purity oxygen is generally corrosive. It is difficult to control the combustion obtained when oxygen gas is injected into a raw energy-bearing formation. This technique has, on occasion, led to fire damage not just at the injection well, but at separate production wells. This leads to a need for obtaining the benefits of high partial pressures of oxygen for in-situ combustion without the foregoing drawbacks.
The reactivity of and associated danger of oxygen in a cryogenic liquid state is far less. There are requirements due to the cryogenic temperatures. This is well understood and has been reduced to practice for decades by using equipment made of nickel alloys, copper alloys, aluminum, and certain design features. Within a petroleum formation, channeling and vaporization of the cryogenic fluid fractures the formation. The gaseous product of this volatilization causes a miscible and/or non-miscible displacement of the oil driving it from an injection borehole in a flood pattern arrangement. U.S. Pat. No. 4,495,993 provides a method for more safely injecting oxygen into boreholes by using such a cryogenic oxygen-containing mixture.
According to U.S. Pat. No. 4,042,026, the most dangerous point along the oxygen flow path is the borehole. This danger could be lessened or eliminated by several means. The very nature of a cryogenic liquid containing oxygen lessens such danger. Also, a fluid with a lower concentration of oxygen or no oxygen may be injected as a pretreatment. There are many gases and liquids which may be injected into the borehole and which, through reaction or displacement, lessen such danger. Another means would be through the limited injection of an oxygen containing gas, causing a limited in-situ burn in the borehole and adjacent energy raw material containing formation.
The cryogenic liquid method of oxygen injection disclosed in U.S. Pat. No. 4,495,993 has gained some acceptance, however, problems have been encountered. The handling of such cryogenic liquids requires special materials which retain their strength at cryogenic temperatures. Such materials are not commonly used in the oil fields. More specifically, the materials at the wellhead or in the well casing are not usually tolerant of ultra-cold temperatures (e.g., the b.p. for oxygen is -182.79.degree. C). Most common forms of steel, for instance, become brittle at cryogenic temperatures. Thus, the method requires extensive replacement or removal of materials at the wellhead and the borehole. The need for these modifications and for specialized equipment makes the cryogenic method expensive and thereby less attractive to the small operator.
The cryogenic method also has less utility in energy-bearing reservoirs that have been water flooded. The majority of U.S. oil reservoirs, including actively producing reservoirs, are water flooded. The injection of cryogenic liquids is hampered in such reservoirs by ice formation within the oil-bearing subterranean formation with consequent blockage of further injection.
It is an object of the present invention to provide methods to safely inject oxygen into energy-bearing reservoirs without overburdensome modifications at the wellhead or in the borehole and without interference due to water flooding.
It is a further object of the present invention to provide seismic events within an energy-bearing geologic formation. The size and distribution of the seismic event being indicative of the richness and distribution of the energy resource.
These and other objects of the present invention will be apparent to those of ordinary skill in the art in light of the present description and appended claims.